Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Late burn combustion events wherein the combustion is later than intended can also lead to pre-ignition combustion events. Specifically, the late combustion can lead to high exhaust manifold pressures and temperatures, as well as higher than intended exhaust residuals, which raises the probability of pre-ignition events.
Various strategies have been developed for reducing pre-ignition onset by late combustion events. One example approach is shown by Fujii et al. in US 2010/0217504. Therein, a valve overlap period is increased as a desired engine torque increases by advancing an intake valve timing and/or retarding an exhaust valve timing. In response to a possibility of cylinder pre-ignition, a rate of advancing the cylinder's intake valve is decreased and/or a rate of retarding the cylinder's exhaust valve is increased, the selection based on the desired engine torque.
However, the inventors herein have identified a potential issue with such an approach. While the approach of Fujii et al. may mitigate pre-ignition in the cylinder undergoing late combustion, the approach may be unable to address pre-ignition in other affected cylinders. Specifically, the late combustion event in the given cylinder may raise the likelihood of pre-ignition in the given cylinder as well as in one or more neighboring cylinders. For example, the late combustion event in the given cylinder may introduce high amount of hot exhaust residuals into one or more adjacent cylinders by forcing open the exhaust valve of the adjacent cylinder(s). The excess hot residual received in the adjacent cylinders can increase their propensity for pre-ignition. In addition, the excess hot residual can increase the amount of fresh cylinder charge inducted (via turbine spinning), leading to a further increase in the propensity for pre-ignition in the adjacent cylinders. As such, pre-ignition events can reduce engine performance and cause engine degradation.
Thus in one example, the above issue may be at least partly addressed by an engine method comprising, in response to exhaust temperature of combustion in a first cylinder being above a threshold, performing a pre-ignition mitigating action in a second cylinder receiving exhaust residuals from the combustion in the first cylinder. In this way, late burn induced pre-ignition events may be reduced.
In one example, a late combustion event in a first cylinder may be determined based on crankshaft acceleration, spark plug ionization current, cylinder pressure data, etc. A second, neighboring cylinder that is most likely to receive exhaust residuals from the first cylinder may then be identified based on the identity of the first cylinder, the firing order of cylinders on the engine, as well as a configuration of the engine's exhaust manifold. In response to the amount of exhaust residuals being more than a threshold and/or an exhaust temperature of combustion in the first cylinder being higher than a threshold, a pre-ignition mitigating action may be performed in the second cylinder to reduce the likelihood of pre-ignition being induced in the second cylinder due to the late burn event in the first cylinder. This may include selective deactivation of fuel injection to the second cylinder. Alternatively, fuel injection to the second cylinder may be enriched for a duration. A degree of the pre-ignition mitigating action (e.g., the degree of richness of the rich fuel injection) may be adjusted to bring an exhaust temperature of combustion in the second cylinder (after receiving the exhaust residuals from the first cylinder) below a threshold temperature. The pre-ignition mitigating action may also be extended to further cylinders. For example, a third cylinder that is most likely to receive exhaust residuals from combustion in the second cylinder may also be enriched, with a lower degree of richness, to reduce the likelihood of pre-ignition in the third cylinder due to hot residuals received in the second cylinder, and so on.
In this way, by enriching or deactivating fuel injection to one or more cylinders likely to (progressively) receive exhaust residuals from late combustion in a neighboring cylinder, a temperature and pressure of the exhaust residuals may be decreased. By reducing the temperature and pressure of the residuals, the likelihood of pre-ignition induced in a cylinder due to receipt of hot exhaust residuals from a late burning cylinder may be decreased. In addition, the likelihood of pre-ignition being induced by the cylinder receiving the hot residuals in further cylinders is also reduced. Overall, engine degradation due to pre-ignition can be mitigated.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.